Combine all script tests into test-scripts

Remove:
    - tidy self test support from `./mach test`
    - `./mach test-idl`

Adds a `./mach test-scripts` command that is responsible for running all
Python script tests. Run this during the CI to catch regressions in
changes to scripts. The WebIDL tests are still *very* slow and there are
from Gecko, so only run them when "-a" is passed meaning all tests.

.github/workflows/linux.yml

@@ -75,6 +75,8 @@ jobs:
         run: python3 ./mach build --release --with-${{ env.LAYOUT }}
       - name: Smoketest
         run: xvfb-run python3 ./mach smoketest
+      - name: Script tests
+        run: ./mach test-scripts
       - name: Unit tests
         if: ${{ inputs.unit-tests }}
         run: python3 ./mach test-unit --release --with-${{ env.LAYOUT }}